Charge ordered vertical transistors

ABSTRACT

A vertical charge ordered transistor is disclosed. A thin charge ordered layer is employed as a tunnel barrier between two electrodes. A gate-induced accumulation of charge destabilizes the charge ordered state around the circumference of the device, opening up a parallel ohmic conduction channel, which leads to an exponential increase in source-drain current. VCOT devices have the potential to exhibit very large on/off ratios, low off-state currents, and sub-threshold slopes below 60 mV/dec.

This invention was made with government support under Grant No.ONR-N00014-11-1-0664 awarded by the Office of Naval Research. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the field of transistors. Inparticular, the present invention is directed to the field of verticalcharge ordered transistors.

2. Description of the Related Technology

Complex oxide heterostructures have been identified as a potentialmaterial platform for the development of novel electronic devices basedon properties present in oxides but absent in conventionalsemiconductors such as metal-insulator transitions, correlatedelectronic phenomena, and ferroelectricity. A central challenge, andpromise, to the growing field of oxide electronics lies not in thereproduction of traditional semiconductor devices with new materials,but rather in the development of novel devices with differentoperational mechanisms and utilities. Examples of such devices includefield effect transistors (FETs) with ferroelectric gate oxides,modulation-doped Mott FETs, and nanoscale FETs that can be written anderased with scanning probe techniques.

Charge ordering, exhibited by a variety of complex oxides, holds promiseas the physical basis for electronics as the charge ordering transitionis accommodated by an abrupt increase in resistivity, occurs atultrafast time scales, opens up a gap in the density of states, and canbe manipulated with relatively small magnetic and electric fields.Additionally, charge ordering is often only stable in narrowcompositional windows that correspond to specific carrierconcentrations, such as 0.5 or 0.33 free electrons or holes per unitcell.

In compounds with narrow phase stability or in materials near thevicinity of a charge ordered to metallic transition, one may expect theapplication of a gate bias that accumulates or depletes a largeconcentration of carriers to destabilize the charge ordered phase,triggering an electric-field controllable insulator-to-metal transition.Previous work has demonstrated gate bias-induced carrier modulations incomplex oxide systems, including materials in the charge ordered phase.

Therefore there is a need in the field to effectively charge ordermaterial for forming transistors, and in particular vertical chargeordered transistors.

SUMMARY OF THE INVENTION

An object of the present invention may be a vertical charge ordertransistor.

Another object of the present invention may be a method of making avertical charge ordered transistor.

An aspect of the present invention may be a vertical charge orderedtransistor comprising: a charge ordered layer; a source layer locatedadjacent the charge ordered layer; a drain layer located adjacent thecharge ordered layer; a gate located adjacent to at least one of thelayers; and a source electrode located on the source layer and a drainelectrode located on the drain layer.

Another aspect of the present invention may be a method of making avertical charge ordered transistor comprising: forming a charge orderedlayer using an adjacent structural imprinting layer; forming a sourcelayer located adjacent the charge ordered layer; forming a drain layerlocated adjacent the charge ordered layer; forming a gate locatedadjacent to at least one of the layers; and placing a source electrodelocated on the source layer and a drain electrode located on the drainlayer.

Still yet another aspect of the present invention may be a method ofmaking a vertical charge ordered transistor comprising: forming a chargeordered layer using an adjacent structural imprinting layer; forming asource layer located adjacent the charge ordered layer; forming a drainlayer located adjacent the charge ordered layer; forming a gate locatedadjacent to at least one of the layers; and placing a source electrodelocated on the source layer and a drain electrode located on the drainlayer.

These and various other advantages and features of novelty thatcharacterize the invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and the objects obtainedby its use, reference should be made to the drawings which form afurther part hereof, and to the accompanying descriptive matter, inwhich there is illustrated and described a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show schematics of the vertical charge order transistorin the off state (A) and on state (B).

FIGS. 2A and 2B are graphs showing the accumulated carrier concentrationas a function of distance from the dielectric/charge order interface;FIG. 2B shows the calculated spatial distribution of the tunnel barrierheight in the charge ordered layer.

FIG. 3A shows the calculated transistor performance and source-draincurrent as a function of gate voltage for three different source-drainbiases with a 3.6 nm barrier width on linear scale.

FIG. 3B shows the same data used in FIG. 3A plotted on a semilog scale.

FIG. 3C shows the source-drain current as a function of gate voltage fora device with a 4.8 nm barrier width.

FIG. 3D shows the sub-threshold swing, S=d V_(G)/d log₁₀(I_(DS)),presented as a function of gate bias.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention employs a method for enhancing charge orderingtransition temperatures and other physical phenomena in materials, andin particular in ABO3 perovskite oxides through the use of an adjacentstructural imprinting layer. The temperature of the charge orderingtransition is strongly coupled to the magnitude of B—O—B bond angles inperovskites. These bond angles are determined by rotations of the BO6octahedra. In bulk perovskites, the magnitude of the BO6 octahedralrotations is determined by the material composition (i.e. elements areon the A-site and B-site).

The magnitude of octahedral rotations is controlled in a material (X) byforming an interface with a thicker second material (Y). At theinterface between X and Y, the octahedral rotations present in materialY will be transferred into material X, forcing X to take on theoctahedral rotations present in material Y. The use of a structuralimprinting layer will allow control of the octahedral rotations in thecharge ordered layer independent of its composition. Therefore,composition and structure may be independently controlled to enhancecharge ordering. The magnitude of octahedral rotations controls a widerange of material properties from optical to electrical to catalytic.Thus, the use of a structural imprinting layer to control octahedralrotations in an adjacent layer is not limited to applications involvingcharge ordering and may find a much broader use.

The use of a structural imprinting layer in order to charge ordermaterial may be used in the design and operating principles of avertical charge ordering transistor. The proposed device may be avertical transistor, in which an active trilayer containing the chargeordered channel layer is grown between source and drain contacts,patterned into a layered structure, such as a mesa, and conformallycoated with a gate oxide and a side gate.

Field-effect biasing may be used to destabilize the charge ordered statevia electrostatic fields or carrier doping, thereby producing an ordersof magnitude change in the channel resistance. This device addresses aprimary weakness of oxide electronics—low carrier mobilities at roomtemperature—by relying on tunneling as opposed to band transport througha channel layer. As the charge ordering phase transition is of purelyelectronic origin, occurring at sub-picosecond time scales, the deviceholds the promise of ultrafast operation. Additionally, charge orderingarises from interactions between nearest neighbor B-site cations. Thelocal nature of these interactions should be conducive to scaling of thedevice down to the nanoscale regime. Thus, the proposed device of thepresent application offers feasible ultra high density with ultrafastcontrol of electron conductivity. Variations of this device design wouldinclude horizontal transistors, the use of current pulses instead of afield-effect to induce the charge order/disorder transition, and sensorsin which light or magnetic fields induce the charge order/disordertransition.

Charge ordering (CO) is a real space ordering of valence electronsdriven by Coulombic electron-electron interactions. Charge orderingholds tremendous potential as a basis for electronic devices. The chargeordering transition from a disordered to ordered state is accompanied byan abrupt increase in resistivity; CO can be melted at ultrafasttimescales; and only weak fields are needed to melt charge ordering,offering the promise of low power devices.

As discussed above, the present invention may employ a static imprintinglayer adjacent to the CO material to engineer the atomic structure inthe CO layer so that the CO layer is susceptible to electric-fieldcontrol of the charge ordered phase transition at room temperature. Bycoupling an electronic and a structural phase transition, ultrafastconductivity changes is united with the temperature stability requiredfor a real-world paradigm changes in electronics.

One promising approach is to utilize the abrupt and large response(electrical, magnetic, caloric, etc) associated with functional phasetransitions as the basis for electronic devices. The ABO₃ perovskiteoxides have been identified as a materials class with which to pursuethis strategy, as they exhibit a wide range of functionalities(magnetism, ferroelectricity, correlated electron phenomena, chargeordering), the energetics of which are often in close competition withone another. Thus, it is possible to tune between different electronicphases using external fields or via changes to the A- and B-site cationcomposition. The functionality of these materials can be furtherenhanced through the formation of coherent heterostructures consistingof isostructural but chemically distinct layers, allowing oxides withdifferent electronic and structural behavior to be brought together atan atomically abrupt, epitaxial interface. Additionally, perovskiteoxides are anticipated to be highly scalable for nanoelectronics, asparts per million levels of doping are not required to tune theirelectronic properties, as in conventional semiconductors.

There are two dominant mechanisms for inducing phase transitions inperovskites; an electronic approach in which the carrier concentrationon the B-site cation is changed or a structural approach in whichchanges to B—O—B bond angles and B—O bond lengths alter either theelectronic bandwidth or the stability of polar distortions. In thelatter case, the bond angles and lengths are determined by the magnitudeof rotations and distortions of the corner sharing BO₆ octahedra.

A method contemplated in the present invention is to use a structuralimprinting layer to form the charge ordered layer. The charge orderedlayer separating the source and drain contacts may then be manipulatedwith an external gate voltage. The proposed device may be that which isshown in FIGS. 1A and 1B, which is a vertical charge ordering transistor(VCOT) 100, discussed in detail below. In the VCOT 100 an active layeredstructure 50 containing the charge ordered layer 10 is grown betweensource layer 20 and drain layer 30, and in the embodiment shown herein,patterned into a mesa, and conformally coated with a gate 40.Field-effect biasing is used to destabilize the charge ordered state viaelectrostatic doping, thereby producing an orders of magnitude change inthe channel resistance. Charge ordering will be stabilized viainterfacial coupling of octahedral behavior between the channel layer 10and adjacent structural imprinting layers. The use of the imprintinglayers raises the structural phase transition temperature associatedwith rotations of the BO₆ octahedra allowing for control of therotational magnitudes, which determines the electronic phase stability(charge ordered or disordered) of the channel layer. By coupling anelectronic and a structural phase transition, ultrafast conductivitychanges and the temperature stability required for a real-world paradigmchange in electronics are united.

Shown schematically in FIG. 1A, the VCOT 100 comprises metallic sourceand drain electrodes 22, 32, and further may include insulating rotationimprinting layers (source layer 20 and drain layer 30), and a chargeordered layer 10. The drain electrode 32, imprinting layers, and chargeordered layer may all be perovskite oxide layers, grown epitaxially on aperovskite substrate 15 such as SrTiO₃.

Materials for suitable imprinting layers (source layer 20, drain layer30), and charge ordered layer, may include ReAlO₃, LaBO₃, and ReFeO₃(Re=rare earth ion, B═Ga, In, Lu) for the imprinting layers due to thelarge octahedral rotations found in these materials.Re_(0.5)Sr_(0.5)MnO₃ may be used for the charge ordered layer sincemanganites exhibit strong tendencies toward CO at half-doped conditionswhere the Mn cations can form a Mn³⁺/Mn⁴⁺ alternating pattern. Changesin carrier concentration that disrupt this 1:1 ratio of Mn³⁺/Mn⁴⁺typically act to destabilize the charge ordered state. Electrostaticdoping via field-effect gating may be used to induce the CO phasetransition by altering the carrier concentration in the channel layer10. Both perovskites and elemental metals may be employed as the sourceelectrode 22. The compound that yields the lowest contact resistance ispreferable. Both LaNiO₃ and La_(0.7)Sr_(0.3)MnO₃ may be used as a sourceelectrode 22 and a drain electrodes 32, since these compounds exhibitmetallic conductivity. These heterostructures will be patterned intolayered structures 50, such as mesas and conformally coated with a gatedielectric 44, which in the embodiment shown may be HfO₂ and a splitside gate electrode 42.

Between the metallic source electrode 22 and drain electrode 32 are theimprinting layers (source layer 20 and drain layer 30) and chargeordered layer 10, which are insulating in the off-state of the device.These insulating layers create a thick (≈4-8 nm) tunnel barrier, whichmay limit the device to very low current and leakage in the off-state.The modulation of channel current may be achieved through themanipulation of the charge ordering phase transition via a field-effectprovided by the gate 40. The melting of charge ordering due toelectrostatic doping reduces the tunnel barrier width from(2d_(I)+d_(CO)) to (2d_(I)), where d_(I) and d_(CO) are the thicknessesof the imprinting and CO layers, producing large changes in conductanceand a sizable on/off ratio due to the exponential dependence oftunneling current on barrier thickness. The effect of the gate bias isagain noted and will alter the carrier concentration in the CO layer,thereby destabilizing the charge order. The gate bias will not alter theoctahedral rotations since their centric nature prohibits coupling to anelectric field.

This VCOT 100 addresses a primary weakness of oxide electronics, whichis low carrier mobilities at room temperature by relying on tunneling asopposed to band transport through a channel layer. As the chargeordering phase transition is a purely electronic transition, occurringat femtosecond time scales, the device holds the promise of ultrafastoperation. Additionally, charge ordering arises from interactionsbetween nearest neighbor B-site cations. The local nature of theseinteractions should be conducive to scaling of the device down to thenanoscale regime. Thus, VCOT 100 offers feasible ultra-high-density withultrafast control of electron conductivity.

An important aspect of the VCOT device 100 is that it is able to takeadvantage of the ability to induce a charge ordered state in a thinperovskite layer and to engineer the temperature (T_(CO)) associatedwith the CO transition to be above 300 K. To accomplish these goals,T_(CO) is controlled by coupling to the structural phase of the adjacentrotation imprinting layers. To understand the approach, half-dopedmanganites exhibit either an insulating Mn³⁺/Mn⁴⁺ charge ordered stateor a metallic ferromagnetic state depending on the average radii of theA-site cations. The reduction of the A-site cation size increases themagnitude of MnO₆ octahedral rotations, reducing electronic bandwidth.In bulk compounds (from which the phase diagram is derived), the solemechanism for altering octahedral behavior is a change in composition.However, in epitaxial heterostructures octahedral behavior can beimprinted in adjacent materials due to interfacial coupling. In otherwords, octahedral behavior exhibits a proximity effect, extending beyondchemically abrupt heterointerfaces. In our heterostructures, theimprinting layers will be designed such that the octahedral rotationsare large enough—i.e., the structural phase transition associated withthe onset of rotations occurs at large enough temperatures—to impart thedesired octahedral behavior to the channel layer and favor charge orderover disorder. T_(CO) and the abruptness of the resistivity change atthe CO transition may possibly be tuned via the interfacial coupling ofthe electronic and structural phases in the channel and imprintinglayers.

Candidate perovskite oxides for the “imprinting layer” are nowdiscussed. The principal goal is to identify the optimal octahedralrotation pattern, with the largest magnitude rotations, that enhancesthe charge-ordered state in the “active” (La,Sr)MnO₃ layer. Thepreferred material properties for this layer describe how they will becomputationally evaluated.

The preferred material should have suitable and robust octahedralrotations. The atomic structure of the perovskite imprinting layershould contain large (in amplitude) and the appropriate symmetry (in- orout-of-phase) octahedral rotations to enhance the CO susceptibility ofthe active layer. The preferred material should be Electricallyinactive. The ideal imprinting layer is a robust (wide band gap)insulator throughout the device operating temperature range. Thisrestriction enforces the VCOT 100 operation to be confined to the activelayer and avoids waste current leakage. The preferred material shouldhave rectifying heterointerfaces. The electronic properties of theimprinting layer/CO-active layer heterointerface should yield a Schottkybarrier. This permits modulation of the carrier concentration of the COlayer through the application of an external bias.

Optimization of the magnitude of the tilt pattern of the materials maybe performed as it has been established that large rotation anglesenhance CO through electronic bandwidth narrowing. Because theparticular influence of different rotational patterns, especially in thepresence of coherent strain, could greatly modify the imprintinglayer-supported charge-ordered state.

The imprinting layers are preferably structurally and electronicallycompatible with the active (La,Sr)MnO₃. Possible materials areinsulating perovskite-structured imprinting layers that preserve theA-site cation sublattice throughout the heterostructure, i.e. La orSr-containing oxides. Possible examples are trivalent La-basedperovskite oxides (LaBO₃), such as LaGaO₃, LaInO₃ and LaLuO₃. The meritof using La on the A-site is its electronic compatibility with the(La,Sr)MnO₃ active layer: Aside from the Sr dopants, the heterointerfaceis free from any polar discontinuity (and internal electrostaticfields).

Another possible material for use are Trivalent aluminates (AAlO₃).Aluminate-based imprinting layers are preferred because Al³⁺ cationsensure the electrical inertness of the imprinting layer. LaAlO₃ may beused by replacing La with smaller A-site cations, such as Sm, Gd, or Dyto enhance the magnitude of the octahedral rotations.

The choices for potential rotational imprinting perovskites may bebroader than the lanthanum- and aluminate-based perovskites outlinedabove. In addition to the materials mentioned above, the Sr and divalentA-site oxide analogues, including B-site transition metals to the abovematerials (needed to avoid the polar catastrophe at SrO-terminated LSMOheterointerfaces), may be explored if our simulations indicate theLa-based and aluminate perovskite oxides are unsuitable imprintinglayers.

An important operating principle of the VCOT 100 relies on theimprinting layer transferring its octahedral rotation pattern (andsymmetry cooperative distortions) into the active charge-ordering layer10, placing the atomic structure of the active layer in close proximityto the functional charge-ordered state. Since the rigidity of the oxygenoctahedral framework in the perovskite structure requires all octahedrain the same plane to rotate in opposite sense their cooperative naturesuggests that octahedral rotations that are favored in one layerinteract strongly with those in the adjacent layers. As a result, theoctahedral rotations should significantly penetrate across theheterointerface. The penetration depth is important for the deviceoperation; for example, the length scale over which the octahedralrotations propagate will in part determine the optimal devicedimensions, namely the thickness ratio of the active layer to theimprinting layer.

Oxide molecular beam epitaxy (MBE) will be utilized to growheterostructures consisting of material compositions and layerthicknesses identified using ab initio computational techniques. MBE ispreferable for the work, as it does not rely on redepositing materialfrom a target, as in sputtering or pulsed laser deposition. Therefore,MBE allows for films with differing compositions to be synthesized in arelatively short period of time. Additionally, the structural quality ofoxide heterojunctions deposited by MBE rival or surpasses thosedeposited by other techniques. The precise control over layering andcomposition made possible with MBE growth is ideally-suited tocollaborative efforts with theorists, as has been demonstrated in recentstudies in which MBE was used to stabilize oxide compounds predicted bytheorists to exhibit multiferroic properties.

While the underlying concepts were discussed above with respect to theVCOT 100, the operating principals of a VCOT 100 are described below.FIGS. 1A and 1B show an exemplary VCOT 100. In the VCOT 100, a thincharge ordered layer 10 is employed as a tunnel barrier between adjacentsource layer 20 and drain layer 30 with metallic conductivity. A gatebias is used to trigger a charge order-to-disorder phase transition inthe charge ordered layer 10 that operates as the barrier around thecircumference of the layered structure 50. The gate bias-inducedreduction and/or removal of the tunnel barrier formed by the chargeordered layer 10, opens up a parallel conduction path, through which anexponential increase in source-drain current can flow. The VCOT 100offers the potential for high on/off ratios, very low sub-thresholdcurrents, and large transconductance values.

The VCOT 100 structure is shown in FIG. 1. While the VCOT 100 describedherein is employs an all-oxide device, the working principles may betransferable to other material platforms that exhibit charge-orderingbehavior. The device consists of a layered structure 50, which is mesashaped in the embodiment shown, containing a bottom conducting oxidelayer that is the drain layer 30, a thin charge ordering layer 10, and atop conducting oxide layer that is the source layer 20.

The layered structure 50 is surrounded by a gate 40 formed by the gatedielectric 44 and gate electrode 42. The application of a positive biasbetween the gate 40 and drain layer 30 (V_(G)), leads to theaccumulation of carriers in the charge ordered layer 10 at the interfacewith the gate dielectric 44. The addition of excess carriersdestabilizes the charge ordering leading to a local phase transitionfrom the ordered to the charge disordered state. With the melting of thecharge ordered state around the mesa circumference, the tunnel barrieris also reduced or removed producing a parallel, low-resistanceconduction path around the edge of the mesa.

Candidate material systems for the source layer 20 and drain layer 30are LaNiO₃, La_(2/3)Sr_(1/3)MnO₃, and SrRuO₃. All of the aforementionedmaterials are oxides with metallic electronic behavior.

Charge ordering is found in a number of perovskite families includingthe manganites (A_(n+1)Mn_(n)O_(3n+1)), ferrites(A_(n+1)Fe_(n)O_(3n+1)), and nickelates (A_(n+1)Ni_(n)O_(3n+1)), wheren=1, 2, or ∞ and A represents either a single cation or combinations ofdivalent and trivalent cations, such as La_(1−x)Ca_(x). In theembodiment discussed above, LaNiO₃ is used to form the source layer 20and drain layer 30. HfO_(x) is used to form the gate dielectric 44. Alis used to form the gate electrode 42, and La_(1/3)Sr_(2/3)FeO₃ is usedto form the charge ordered layer 10.

An example of the proposed source/channel/drainheterostructure—LaNiO₃/La_(1/3)Sr_(2/3)FeO₃ (9-12 unit cells)/LaNiO₃(>30 unit cells)—can be synthesized using either oxide molecular beamepitaxy, pulsed laser deposition, or sputtering. Atomic layer depositionis one process that can be used to conformally coat the mesaheterostructure with HfO_(x) and the gate metal.

In the example discussed above and used in the calculations presentedhere, the layered structure 50 has a diameter of 100 nm, the thicknessof the gate dielectric 44 is 10 nm, and the operating temperature is 300K. Optimization of the device may involve the selection of geometricparameters that include a range of mesa diameters from 10-2000 nm andconcomitant thicknesses of the gate dielectric 44 from 2 to 50 nm.

In the off-state of the VCOT device 100 (V_(G)=0), the source-draincurrent (I_(DS)) is determined by the tunnel barrier height and width.I_(DS) decreases exponentially with increasing barrier width, thus verylow off-state currents can be achieved. Calculating the tunnelingcurrent using a rectangular barrier potential, values for the I_(DS) are4.9×10⁻¹¹ and 1.2×10⁻¹³ A, assuming a source-drain voltage (V_(DS)) of100 mV, a barrier height of 0.25 eV, and barrier widths (t) of 3.6 nmand 4.8 nm, respectively.

The application of a gate bias leads to a charge accumulation at theinterface between the charge ordered layer 10 and the gate dielectric44. The charge and potential as a function of distance are related viaPoisson's equation; for the case of a cylindrical mesa this relation isgiven by:

$\begin{matrix}{{{\nabla_{r}^{2}\psi} = {{\frac{\mathbb{d}^{2}\psi}{\mathbb{d}r^{2}} + {\frac{1}{r}\frac{\mathbb{d}\psi}{\mathbb{d}r}}} = {\frac{kT}{q}{\delta\mathbb{e}}^{\frac{q\;\psi}{kT}}}}},} & (1)\end{matrix}$

with δ=q²n_(i)/kT∈_(r)∈₀, where ψ is the potential, ∈_(r) is therelative dielectric constant of the charge ordered layer 10 (taken hereto be 20), ∈₀ is the permittivity of free space, kT is the thermalvoltage, n_(i), is the intrinsic carrier concentration (assumed here tobe 10¹⁹ cm⁻³ in the charge ordered state), and q is the electron charge.The general solution for the potential from eqn (1) is:ψ(r)=A−2 ln(Br ²+1)  (2)

where A and B are constants determined from the boundary conditions thatthe derivative of the potential is zero at the center of the mesa andthat the value of potential at the gate dielectric channel interface beequal to the surface potential, ψ(r₀)=ψ_(S). This equation provides anexpression for the potential as a function of radial distance into thecharge-ordered oxide channel but requires the value of surface potentialto be related to the applied gate voltage. To accomplish this, thecharge on the gate electrode 42 is set equal to that at the channel-gatedielectric interface:

$\begin{matrix}{{{C_{ox}( {V_{GS} - {\Lambda\phi} - \psi_{S}} )} = {ɛ_{r}ɛ_{0}\frac{kT}{q}\sqrt{\frac{32}{t^{2}}( {1 + {\frac{1}{16}\delta\; t^{2}{\mathbb{e}}^{\frac{q\;\psi_{S}}{kT}}} - \sqrt{1 + {\frac{1}{8}\delta\; t^{2}{\mathbb{e}}^{\frac{q\;\psi_{S}}{kT}}}}} )}}},} & (3)\end{matrix}$

where C_(ox) is the oxide capacitance, V_(GS) is the applied gatevoltage, Δ□ is the gate metal-channel work function difference, ψ_(S)isthe surface potential and t is the thickness of the channel.¹⁹ Solvingfor ψ_(S) provides a relationship between the constant B and ψ_(S):

$\begin{matrix}{{\mathbb{e}}^{\frac{q\;\psi_{S}}{kT}} = {- {\frac{8B}{{\delta( {{Br}_{0}^{2} + 1} )}^{2}}.}}} & (4)\end{matrix}$

From this, the constant A can be found via:

$\begin{matrix}{A = {\ln( {- \frac{8B}{\delta}} )}} & (5)\end{matrix}$

Note that the constant A sets the potential to an offset value at thecenter of the mesa. This pinning of the potential is a result of thesurrounding-gate geometry but if the radius is large (approximatelygreater than 50 nm), this value goes to zero—i.e., the potential at thecenter of the mesa goes to zero as in the limiting case of single-gatetransistors.

Once the potential ψ(r) is determined, the spatial dependence of theaccumulated charge is then be solved from:

$\begin{matrix}{n = {n_{i}{\mathbb{e}}^{\frac{\psi_{r}}{kT}}}} & (6)\end{matrix}$

FIG. 2A shows the calculated carrier concentration within the chargeordered layer 10 as a function of distance of the gate dielectric 44 fora variety of gate biases. The accumulated charge destabilizes the chargeordered phase reducing the tunnel barrier height, and above a criticalcarrier concentration removing the tunnel barrier. Here it is assumedthat the charge order to disorder transition is complete, and thereforethe tunnel barrier is removed, with the accumulation of 0.1 excesscarriers per unit cell. For accumulated charge less than this criticalvalue, the tunnel barrier is assumed to decrease in a linear manner withincreasing accumulated carrier concentration. The barrier height in eachunit cell near the gate dielectric/charge ordered layer interface isdetermined from integrating the accumulated charge profile shown in FIG.2A over each unit cell to obtain the spatially resolved charge per unitcell. The obtained barrier height profiles are shown in FIG. 2B for avariety of gate biases.

The source-drain current as a function of gate bias is modeled by usinga parallel resistor model, in which I_(DS) is calculated for a series ofparallel conduction channels. For the unit cells in which the tunnelbarrier completely removed, the resistance is simply >t/A, where ρ isthe resistivity of the material in the charge disordered state (hereassumed to be 0.01 ^-cm), t is the channel length (same as the barrierwidth), and A is the area of the region over which the barrier iscompletely removed. For areas in which the barrier height is reduced butnot removed, the resistance (R) is calculated from standard tunnelingequations. Thus, the drain current can be solved by combining theresistors as:

$\begin{matrix}{{R_{eq}^{- 1} = {\frac{1}{R_{1}} + \frac{1}{R_{2}} + \frac{1}{R_{3}} + \ldots + \frac{1}{R_{n}}}},} & (7)\end{matrix}$

with the value of resistances proceeding from the first unit cell at thechannel-gate dielectric interface inward and the resistance R_(n), beingthe bulk resistance of the channel with the unmodified tunnel barrierheight and n being the number of unit cells that it takes for thebarrier height to return to its original value. This model then allowsthe drain current to be found analytically using Ohm's law. Note that arigorous description of the non-bulk-like transport characteristics ofthe charge ordered layer 10 within the nanoscale mesa may be necessaryfor a more precise model of the source-drain current as a function ofgate bias, for instance to fully describe finite thicknesses effects inthe melted portion of the charge ordered layer 10; such detailedmodeling is beyond the state-of-the art. The device modeling approachutilized here captures the essential physics and performance metrics ofthe VCOT 100.

The results from this model are shown in FIG. 3. The source-draincurrent is found to increase exponentially for gate biases less than 0.1V, resulting from a reduction and removal of the tunnel barrier aroundthe mesas circumference with increasing gate bias. While the on-statecurrent is set by the resistivity of the channel layer in its chargedisordered state, very large on/off ratio can be achieved by choice ofthe barrier width, which controls the off-state current. For instance,on/off ratios of ˜10⁷ and 10⁹ are calculated for barrier widths of 3.6and 4.8 nm. In addition to the excellent on/off ratios, the VCOTexhibits an extremely rapid increase in I_(DS) as a function of V_(G).This behavior can be quantified by calculating the subthreshold swing,S, defined as S=dV_(G)/d log₁₀(I_(DS)). While the minimum possible valueof S in conventional MOSFETs operating at room temperature is 60 mV/dec,values as small as 7 mV/dec are calculated for the VCOT in the gate biasregimes that trigger the charge order/disorder transitions. Such lowvalues of S are a clear advantage of the VCOT 100 compared to existingtransistors. In the modeled device, this minimum value of S occurs atV_(G)=0.05 V. For gate biases less than this, S begins to increase asnot enough charge is accumulated to melt the tunnel barrier. For gatebiases greater than this, S is orders of magnitude larger as I_(DS)begins to saturate. The calculated S values are shown in FIG. 3D as afunction of gate bias. A maximum transconductance (=dI_(DS)/dV_(G))value of 2 mA/V was obtained near V_(G)=75 mV for the device with t=3.6nm and V_(DS)=150 mV. Note that the calculations for accumulated chargerepresent the largest values that could be physically expected. Inactual devices, defects would be expected to reduce the amount of freecharge accumulated for a given gate bias. Therefore, it is anticipatedthat in working devices the minimum S values and device turn-on wouldoccur at higher gate voltages, likely between 0.5 and 5 V.

The VCOT 100 has geometric similarities with semiconductor-based p-i-nvertical tunneling transistors, which also employ a gate wrapped arounda mesa structure containing the channel (tunnel barrier) layer. Whileboth devices are based on carriers tunneling through the channel layerand thus have very low sub-threshold currents, the operating principlesare fundamentally different. In semiconductor vertical tunnelingtransistors, the gate voltage reduces the barrier width for carriers totunnel between the valence band of the p-type layer and conduction bandof the n-type layer. Thus, the on-state state current is limited bytunneling. In the VCOT 100, the introduction of carriers in the chargeordered channel modulates the phase of the material (removes the chargeordered state), causing an insulator to metal transition. Thus, the gatevoltage acts to remove the tunnel barrier between two metallicelectrodes, source electrode 22 and drain electrode 32.

The VCOT device 100 described herein will motivate further developmentof charge ordered oxides and heterostructures in which charge orderedmaterials are utilized as tunnel barriers. In particular, transportstudies across heterojunctions of metallic oxides and charge orderedoxides are needed to obtain accurate values of tunnel barrier heights.Studies of gate-induced charge order/disorder phase transitions areneeded to better understand how the band gap is reduced with theintroduction of excess carriers. Additionally, the device 100 motivatesthe search of new charge ordered materials with large energy gaps.

In summary, the operating principles for a new electronic device inwhich a charge ordered compound is utilized as a tunnel barrier betweenmetallic electrodes, source electrode 22 and drain electrode 32. Theapplication of a gate bias destabilizes the charge ordered state,leading to a rapid increase in source-drain current. The main advantagesof this VCOTT device 100 are a large on/off ratio, very low off-statecurrents, and sub-threshold slope values well below 60 mV/dec.

Charge ordering holds tremendous potential as a basis for electronicdevices as the charge ordering transition from a disordered to orderedstate is accompanied by an abrupt increase in resistivity; CO can bemelted at ultrafast timescales; and only weak fields are needed to meltcharge ordering, offering the promise of low power devices. Thedifference between the high and low resistance state offers a naturalfoundation for binary logic operations and data storage. Therefore, itis anticipated that charge ordering based transistors will be faster andoperate at lower power than conventional silicon transistors.

The use of interfacial octahedral coupling to control bond angles inperovskites allows for both the structure of perovskites to beengineered independently of their cation composition. The ability todecouple structure and composition is not feasible in bulk compounds, inwhich octahedral behavior is determined by the material composition;therefore, the structural coupling of octahedral behavior offers aunique route to control the structure in complex oxides.

The VCOT 100 represents a new electronic device paradigm, impossible toreplicate with silicon-based physics. The operating principles of thedevice lend themselves to scalability to nanometer sizes, with lowoperating voltages. While beyond the scope of the current proposal, thedevice may be made to exhibit non-volatile operation by replacing theHfO₂ gate dielectric with a ferroelectric oxide, which can be depositedusing ALD. The demonstration of room temperature charge ordering in adevice geometry could also be used as a basis for high precisionmagnetic field sensors, as magnetic fields are known to melt chargeordering. In such devices, the abrupt transition from an insulating toconducting state that accompanies the melting of charge ordering wouldbe used to sense external fields. The high/low resistance state may alsobe used for memory devices. The wide range of applicability of thesedevices—logic, sensing, memory—could provide a novel platform in which asingle device architecture is used for multiple ultrafast functions,providing the Navy with superior electronic capabilities over potentialadversaries.

The advances made in the understanding of oxide heterostructure design,epitaxy, and characterization are likely to impact a range oftechnologies, not limited to electronics. Oxides are also used fortransducers, energy conversion and storage devices, catalysis, andcoatings. For all these applications, the ability to synthesize highquality oxide films with engineered surfaces is crucial to optimizingperformance. Additionally, the next generation of technologies will relyon new materials, in particular heterojunctions or superlattices thatcombine multiple functionalities into a single structure. To realizesuch materials, it is crucial to understand the relationship betweenatomic and electronic structures to establish design criteria and theprocesses that occur in oxide film growth. By investigatingstructure/processing/properties relationships of the proposed materials,this project will advance the Navy's capabilities of fabricating novelfunctional oxide heterostructures.

An alternative embodiment may be a horizontal charge orderingtransistor. This device will consist of the imprinting/CO/imprintingtrilayer with source and drain electrodes contacting the CO layer butseparated in the lateral direction. A dielectric oxide and a metal gatewill be deposited between the source and drain electrodes. Geometricallysimilar to conventional FETs, this device will be used to studyfield-effect manipulation of charge ordering before moving on to themore technically challenging vertical geometry

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

What is claimed is:
 1. A vertical charge ordered transistor comprising:a charge ordered layer; a source layer located adjacent the chargeordered layer; a drain layer located adjacent the charge ordered layer;a gate located adjacent to at least one of the layers; and a sourceelectrode located on the source layer and a drain electrode located onthe drain layer.
 2. The vertical charge ordered transistor of claim 1,wherein the gate comprises a gate electrode and a gate dielectric. 3.The vertical charge ordered transistor of claim 1, wherein the sourcelayer is made from a perovskite material.
 4. The vertical charge orderedtransistor of claim 3, wherein the drain layer is made from a perovskitematerials.
 5. The vertical charge ordered transistor of claim 3 whereinthe source layer material is selected from the group of materialsconsisting of LaNiO₃, La_(2/3)Sr_(1/3)MnO₃, and SrRuO₃.
 6. The verticalcharge ordered transistor of claim 5, wherein the drain layer materialis selected from the group of materials consisting of LaNiO₃,La_(2/3)Sr_(1/3)MnO₃, and SrRuO₃.
 7. The vertical charge orderedtransistor of claim 1, wherein the charge ordered layer is made fromLa_(1/3)Sr_(2/3)FeO₃.
 8. A method of making a vertical charge orderedtransistor comprising: forming a charge ordered layer using an adjacentstructural imprinting layer; forming a source layer located adjacent thecharge ordered layer; forming a drain layer located adjacent the chargeordered layer; forming a gate located adjacent to at least one of thelayers; and placing a source electrode located on the source layer and adrain electrode located on the drain layer.
 9. The method of claim 8,wherein the gate comprises a gate electrode and a gate dielectric. 10.The method of claim 8, wherein the source layer is formed from aperovskite material.
 11. The method of claim 10, wherein the drain layeris formed from a perovskite materials.
 12. The method of claim 10,wherein the source layer material is formed from the group of materialsconsisting of LaNiO₃, La_(2/3)Sr_(1/3)MnO₃, and SrRuO₃.
 13. The methodof claim 11, wherein the drain layer material is formed from the groupof materials consisting of LaNiO₃, La_(2/3)Sr_(1/3)MnO₃, and SrRuO₃. 14.The method of claim 8, wherein the charge ordered layer is made fromLa_(1/3)Sr_(2/3)FeO₃.
 15. A vertical charge ordered transistorcomprising: a charge ordered layer; a source layer located adjacent thecharge ordered layer; a drain layer located adjacent the charge orderedlayer; a gate located adjacent to at least one of the layers; a sourceelectrode and a drain electrode; and wherein the source layer and thedrain layer imprint the charge ordered layer.
 16. The vertical chargeordered transistor of claim 15, wherein the gate comprises a gateelectrode and a gate dielectric.
 17. The vertical charge orderedtransistor of claim 15, wherein the source layer is made from aperoskovite material.
 18. The vertical charge ordered transistor ofclaim 17, wherein the drain layer is made from a peroskovite materials.19. The vertical charge ordered transistor of claim 17, wherein thesource layer material is selected from the group of materials consistingof LaNiO₃, La_(2/3)Sr_(1/3)MnO₃, and SrRuO₃.
 20. The vertical chargeordered transistor of claim 19, wherein the drain layer material isselected from the group of materials consisting of LaNiO₃,La_(2/3)Sr_(1/3)MnO₃, and SrRuO₃.
 21. The vertical charge orderedtransistor of claim 15, wherein the charge ordered layer is made fromLa_(1/3)Sr_(2/3)FeO₃.